CH 38

Replacement Surgery and Prosthetics

Replacement surgeries for joints, notably hips and knees, are among the most frequently performed. Additionally, prosthetic joints for shoulders, elbows, and fingers are also available. Despite advancements in these surgical techniques, improvements in prosthesis design are still necessary. Current state-of-the-art prostheses exhibit limited durability, particularly wearing out quickly in young or highly active individuals. Research efforts are currently concentrated on employing new materials, such as carbon fiber, which may enhance the longevity and durability of these prosthetic devices.

Types of Skeletal Systems

Overview

By the end of this section, you will be able to:

  • Discuss different types of skeletal systems
  • Explain the role of the human skeletal system
  • Compare and contrast different skeletal systems
    The skeletal system is crucial for supporting the body, protecting internal organs, and facilitating movement. Three primary skeletal designs perform these functions: hydrostatic skeletons, exoskeletons, and endoskeletons.

Hydrostatic Skeleton

A hydrostatic skeleton consists of a fluid-filled compartment known as the coelom. This structure supports organs with the aqueous fluid which resists external compression. The fluid is maintained under hydrostatic pressure, providing support to the organism's organs. This type of skeletal system is prevalent in soft-bodied animals, including sea anemones, earthworms, and Cnidaria, as well as various invertebrates.

Movement Mechanism

Movement within a hydrostatic skeleton is executed by muscles that encircle the coelom. As these muscles contract, the shape of the coelom changes, causing the fluid pressure to generate movement. For example, earthworms exhibit movement via peristalsis, where successive waves of muscular contractions shorten and lengthen the body to facilitate locomotion.
While effective for invertebrates and some aquatic organisms, hydrostatic skeletons are not optimal for terrestrial animals.

Exoskeleton

An exoskeleton is defined as an external skeleton that forms a hard encasement on the organism’s surface. Shells from crabs and insects exemplify exoskeletons. This structure offers protection against predators, provides body support, and allows movement through the contraction of attached muscles.

Composition

For arthropods (e.g., crabs, lobsters), exoskeletons are composed of 30–50% chitin, a strong yet flexible polysaccharide derivative of glucose, secreted by epidermal cells. The exoskeleton's strength is further enhanced by calcium carbonate in species like lobsters. As exoskeletons are acellular, arthropods must molt periodically, shedding their exoskeleton to accommodate growth since the exoskeleton itself does not grow.

Endoskeleton

An endoskeleton is a skeleton composed of hard, mineralized structures situated within the soft tissue of organisms. Primitive examples include the spicules of sponges, while vertebrates have bones formed from true tissue. The endoskeletal structure provides support, protects internal organs, and enables movement through muscle contraction associated with the skeleton.

Human Skeleton

The human endoskeleton comprises 206 bones in an adult and serves five essential functions:

  1. Support to the body.
  2. Mineral and lipid storage.
  3. Hematopoiesis (blood cell production).
  4. Protection of internal organs.
  5. Facilitation of movement.

The human skeletal system is divided into two main parts:

  • Axial Skeleton: Comprising the skull, vertebral column, and rib cage.
  • Appendicular Skeleton: Consisting of limb bones, shoulder girdle, and pelvic girdle.

Human Axial Skeleton

The axial skeleton forms the central axis of the body, encompassing:

  • Bones of the skull
  • Ossicles of the middle ear
  • Hyoid bone of the throat
  • Vertebral column
  • Thoracic cage (rib cage)
Function

The axial skeleton serves to support and protect the brain, spinal cord, and ventral body cavity organs; it also provides attachment sites for muscles involved in head, neck, and trunk movements along with respiratory functions and stabilization of the appendicular skeleton.

Skull

The skull comprises 22 bones and is subdivided into two categories:

  1. Cranial Bones: Eight bones forming the cranial cavity, which protects the brain and provides attachment sites for head and neck muscles. Notable cranial bones include:
       - Frontal bone
       - Parietal bones (x2)
       - Temporal bones (x2)
       - Occipital bone
       - Sphenoid bone
       - Ethmoid bone

       These bones fuse tightly in adults, having initially developed separately during embryonic stages.

  2. Facial Bones: Fourteen bones forming the face. These bones also create cavities for sensory organs and protect respiratory and digestive tracts. The facial bones consist of:
       - Nasal bones (x2)
       - Maxillary bones (x2)
       - Zygomatic bones (x2)
       - Palatine bones (x2)
       - Vomer bone
       - Lacrimal bones (x2)
       - Inferior nasal conchae (x2)
       - Mandible

       All facial bones occur in pairs except for the mandible and vomer.

While not part of the skull, the hyoid bone underlies it, providing a movable base for the tongue and connecting to the jaw, larynx, and tongue muscles. The mandible articulates with the skull's base, controlling airway and digestive tract openings.

Vertebral Column

The vertebral column, or spinal column, encases and protects the spinal cord, supports the head, and serves as a muscle and rib attachment site. The adult vertebral column consists of 26 bones:

  • 24 vertebrae
  • Sacrum
  • Coccyx
Composition and Characteristics
  • Adult sacrum is typically five fused vertebrae.
  • Coccyx usually comprises 3 to 4 fused vertebrae, with the sacrum and coccyx commonly fusing around age 70.
    Comprising 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, these segments play a critical role in movement and structural stability.

Each vertebral body contains a central foramen allowing for spinal cord passage while notches at either side facilitate spinal nerve exits. The vertebral column averages 71 cm (28 inches) in adult males and possesses a natural curvature observable from a lateral view, enhancing strength and flexibility. The curvature aids in shock absorption, akin to a spring.

Intervertebral Discs

Intervertebral discs composed of fibrous cartilage exist between adjacent vertebrae from the cervical vertebrae to sacrum, enabling spine movement and absorbing shock during dynamic activities. These discs function somewhat like ligaments to stabilize vertebrae together.

Thoracic Cage

The thoracic cage, also known as the ribcage, comprises:

  • Ribs
  • Sternum
  • Thoracic vertebrae
  • Costal cartilages
Protective Functions

The thoracic cage encases and safeguards organs within the thoracic cavity such as the heart and lungs. It additionally supports shoulder girdles and upper limbs, acting as an attachment point for the diaphragm, various back, neck, and chest muscles. Changes in thoracic volume aid respiration.

The sternum, or breastbone, consists of three fused bones in the adult and is located at the anterior section of the chest.
The ribs, notably 12 pairs, are long, curved bones that attach to the thoracic vertebrae, and their anterior ends connect to the sternum through costal cartilages. The lower two pairs (11 and 12) of ribs function as free-floating ribs.

Human Appendicular Skeleton

The appendicular skeleton comprises the bones of the upper limbs, functioning in grasping and manipulating objects, and the lower limbs facilitating locomotion, inclusive of the:

  • Pectoral girdle: Attaches the upper limbs to the axial skeleton.
  • Pelvic girdle: Attaches the lower limbs to the axial skeleton.

Pectoral Girdle

The human pectoral girdle supports upper limb attachment to the axial skeleton, composed of:

  • Clavicle (collarbone)
  • Scapula (shoulder blades)
Clavicle

Clavicles are S-shaped, providing arm positioning and lying horizontally across the chest, just above the first rib. They are quite fragile and susceptible to fractures, particularly during falls when the arms are outstretched. The clavicle articulates with both the sternum and scapula.

Scapula

The scapulae are flat, triangular bones located posteriorly, supporting muscles crossing the shoulder joint. The spine of the scapula demonstrates robust muscle attachment areas, easily palpable through the skin.

Upper Limb

The upper limb is comprised of 30 bones grouped into three regions:

  • The arm (from shoulder to elbow) consisting of the humerus.
  • The forearm composed of the ulna and radius.
  • The wrist and hand encompassing 8 carpal bones, 5 metacarpal bones, and 14 phalanges.
Humerus

The humerus is the largest bone in the upper limb, articulating with the scapula at the shoulder joint and with the forearm at the elbow.

Forearm

The forearm includes the ulna (on the medial side) and radius (on the lateral side). The radius allows for greater rotational movement at the wrist.

Hand

The hand is structured with carpal bones forming the wrist, metacarpals forming the palm, and phalanges composing fingers.

Pelvic Girdle

The pelvic girdle connects the lower limbs to the axial skeleton, evidencing a robust attachment to bear body weight and facilitate movement via deep sockets and strong ligaments securing the femur.

Anatomy of the Pelvic Girdle

  • Consists of two large hip bones (coxal bones), formed by three fused pairs of bones: ilium, ischium, and pubis.
  • The pelvic structure fuses at the anterior pubic symphysis and posteriorly connects to the sacrum.
Sex Differences

The female pelvis exhibits distinct adaptations for childbirth, being lighter, wider, shallower, and presenting a broader pubic angle compared to the male pelvis.

Lower Limb

The lower limb comprises:

  • Thigh, consisting of the femur.
  • Leg, containing tibia and fibula.
  • Foot, incorporating tarsals, metatarsals, and phalanges.
Bone Strength and Structure

Bones of the lower limbs are denser and stronger compared to upper limbs to support body weight during locomotion. Notably, the femur is recognized as the longest and strongest bone in the body.

Key Concept: Changes in Skeletal Design for Land Locomotion

Given the transition from aquatic to terrestrial environments, numerous adaptations occurred in vertebrates' body designs. Such transformations involve changes in the axial skeleton's structure to counteract gravity’s effects and promote efficient locomotion on land.

  • The vertebral column adapted to reduce torsional strain while bearing the weight of the body, developing additional strengthening through ossification.
  • Limbs evolved positioning under the body, allowing for more efficient forward movement via reduced adductor muscle size, promoting a pendulum-like action while walking.

Bone Structure

Bone or osseous tissue is a specialized connective tissue that constitutes the endoskeleton, comprising specialized cells and a mineral matrix predominantly of hydroxyapatite derived from calcium phosphate.

Calcification Process

Calcification involves mineral salt deposition within the collagen matrix, hardening the tissue—though this occurs only when collagen fibers are present. Human bones are categorized by shape into long, short, flat, sesamoid, and irregular bones:

Bone Classification

  • Long Bones: Longer than they are wide, typically possessing a shaft and two ends. Examples include femur, tibia, ulna, and radius.
  • Short Bones: Cube-shaped bones seen in the wrist (carpals) and ankle (tarsals).
  • Flat Bones: Broad and thin, designed for protection and muscle attachment; includes the sternum, ribs, scapulae, and cranial roof.
  • Irregular Bones: Bones with complex shapes like vertebrae and hip bones.
  • Sesamoid Bones: Small, seed-shaped structures, with the patella as a primary example, typically found within tendons.

Bone Tissue Types

Bone tissue comprises compact and spongy bone, which differ in function and structure:

Compact Bone

Forms the hard exterior layer surrounding the medullary cavity, providing structural strength. It consists of microscopic units called osteons or Haversian systems, which are elongated cylinders containing mineral matrices intertwined with living osteocytes and canaliculi. Osteons are arranged parallel to the long bone's axis allowing for effective stress resistance.

Spongy Bone

Located within bones, spongy bone lacks the dense structure of compact bone, instead forming a network of trabeculae that house red bone marrow. It serves to reduce bone density, providing softer, more flexible support that enables shock absorption in various anatomical locations.

Bone Cell Types

Bone comprises four cell types:

  1. Osteoblasts: Responsible for the formation of bone tissue. They secrete inorganic and organic parts of the bone matrix.
  2. Osteoclasts: Large, multi-nucleated cells that dissolve bone matrix for remodeling or calcium release.
  3. Osteocytes: Mature bone cells maintaining tissue health and homeostasis.
  4. Osteoprogenitor Cells: Stem cells vital for bone repair and regeneration.

Bone Development and Growth

Ossification (osteogenesis) is the process of bone formation initiated by osteoblasts and encompasses stages such as intramembranous and endochondral ossification.
Bone growth persists until approximately age 25 but remodels throughout life; it adapts and responds to mechanical stress, increasing in strength and density where required while losing density in disused areas.

  • Intramembranous Ossification: Involves the transformation of fibrous membranes into flat bones (e.g., skull bones).
  • Endochondral Ossification: Occurs from hyaline cartilage templates into long bones.
Long Bone Growth Mechanisms

Long bones elongate via cartilage growth at the epiphyseal plate, while appositional growth increases diameter through the addition of bony tissue—they continue lengthening until the plates close, resulting in the epiphyseal line.

Bone Remodeling and Repair

Bone remodeling involves a continuous cycle of deposition and resorption, supported by dietary vitamins (D, C, and A) and minerals (calcium, phosphorus, magnesium).
The process of bone fracture repair consists of sequential stages including:

  1. Hematoma formation at the injury site.
  2. Fibrocartilaginous Callus developing through fibroblast and osteoblast activity.
  3. Bony Callus formation as fibrocartilage transforms into bough by osteoblasts.
  4. Remodeling of bone through osteoclast and osteoblast action to restore original structure over time.

Joints and Skeletal Movement

A joint or articulation is the meeting point between two or more bones, facilitating movement and providing stability. Joints are classified structurally (based on composition) or functionally (based on mobility).

Structural Classification
  • Fibrous Joints: Composed of connective tissue; typically immovable. Includes sutures (skull), syndesmoses (e.g., adjacent tibia and fibula), and gomphosis (tooth to socket connections).
  • Cartilaginous Joints: Bones linked via cartilage; includes synchondroses (growth plates) and symphyses (vertebrae joints).
  • Synovial Joints: Feature synovial cavities allowing free movement, enveloped by articular capsules. Examples include knees and elbows.
Functional Classification
  • Synarthroses: Immovable joints (sutures and gomphoses).
  • Amphiarthroses: Slightly movable joints (syndesmoses and symphyses).
  • Diarthroses: Freely movable, classified as synovial joints.

Movements of Synovial Joints

Movements facilitated by synovial joints can be classified as:

  • Gliding: Flat surfaces moving against each other, e.g., in carpal bones.
  • Angular Movements: Includes flexion, extension, abduction, adduction, and circumduction.
  • Rotational: Involves movement around an axis (medial/lateral).
  • Special Movements: Include inversion, eversion, protraction, retraction, elevation, depression, dorsiflexion, plantar flexion, and opposition in the thumb.
Types of Synovial Joints
  • Planar: Allow gliding (e.g., carpal bones).
  • Hinge: Enable uniaxial movement (e.g., elbow).
  • Pivot: Allow rotation (e.g., neck).
  • Condyloid: Enable angular movement along two axes (e.g., wrist).
  • Saddle: Allow greater range of motion (e.g., thumb).
  • Ball-and-Socket: Allow the most extensive movement (e.g., shoulder and hip).
Rheumatology Context

Rheumatologists specialize in diagnosing and treating joint, muscle, and bone disorders such as osteoarthritis, rheumatoid arthritis (RA), which primarily affects synovial joints. Diagnosing RA involves clinical evaluation backed by imaging and blood tests, with treatment including rest, medications, and potentially surgery for severe cases.

Muscle Contraction and Locomotion

Muscles enable movements, performing vital bodily processes. Muscle tissue comprises three types:

  1. Skeletal Muscle: Voluntary control, striated appearance, responsible for locomotion.
  2. Cardiac Muscle: Found in the heart, involuntary with striated fibers, responsible for pumping blood.
  3. Smooth Muscle: Involuntary, non-striated, lines hollow organs.

Skeletal Muscle Fiber Structure

Skeletal muscle fibers, large in scale, possess a plasma membrane (sarcolemma) and consist of multiple myofibrils running alongside, organizing them into units of sarcomeres defined by Z discs, A bands, I bands, and H zones.

Sliding Filament Model of Contraction

Muscles contract via a mechanism where the sliding of myosin and actin filaments facilitates sarcomere shortening without actual filament length change. ATP plays a crucial role in providing energy for the contraction process.

Regulatory Proteins

In resting conditions, tropomyosin inhibits actin's binding sites. Calcium ions released upon stimulation enable myosin interaction with actin by displacing tropomyosin, facilitating muscle contraction until action potentials cease.

This is a detailed overview of the skeletal and muscular systems, articulating the foundational aspects necessary for the comprehension of human anatomy and physiology related to movement and skeletal support.